check the vibrate motor pins haven't been flattened when you opened phone, if they aren't making contact with underside of motherboard gently prise them up a little
clean contacts on vibrate/motherboard
check the 2 small metal prongs on metal housing of motor are in the slots of rear case
check small plastic switch is seated in slot of mute on/off switch.
failing that you may have to replace connector 6 or vibrate motor
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